1. Field of the Invention
The present invention relates to a synchronous motor and in particular to a rotor structure of the synchronous motor.
2. Description of the Related Art
In a synchronous motor, a torque generation thereof is determined by a gap between an outer periphery of a central portion of each pole of the rotor and an inner periphery of a stator. The torque generation increases as the gap is set smaller. Inductance of the synchronous motor is determined by a gap between an outer periphery of side portions, i.e., regions between the central portion and circumferential ends of each pole of the rotor and the inner periphery of the stator. The inductance is reduced as the gap is set larger. With the reduce of the inductance, a counter electromotive force produced in a high speed rotation is made smaller to produce an advantageous effect of increasing an output torque (power). Further, the energy efficiency is improved when the inductance is reduced.
As shown in FIGS. 7 and 8, a conventional rotor of the synchronous motor has an outer periphery of a circle or a shape defined by a combination of circular arcs in a cross section perpendicular to an axis of a rotor. FIG. 7 shows a conventional rotor of the synchronous motor having the circular outer periphery. An outer periphery of each pole of the rotor with magnets 1 embedded therein is defined by a circular arc and a whole outer periphery of the rotor is defined by connecting circular arcs for the predetermined number of poles to form a circle. A shaft 2 is arranged at a center of the rotor. In this example of FIG. 7, the circular arcs defining respective outer peripheries of the magnets for the predetermined number of poles are arranged on a circle having its center coinciding with a central axis of the rotor (a central axis of the shaft 2) to define the whole outer periphery of the rotor to be the circle.
In order to increase the output torque of the synchronous motor using the rotor as shown in FIG. 7, it is conceived that a radius of the circle formed by the combination of circular arcs for respective poles is made larger so as to decrease a gap between a central portion xe2x80x9caxe2x80x9d of the rotor and an inner periphery of a stator. However, gaps between the side portions xe2x80x9cbxe2x80x9d and the inner periphery of the stator are made smaller to increase the inductance resulting in decrease of the output torque at high speed rotation. On the other hand, in order to decrease the inductance, it is conceived that the radius of the circular arc is made smaller to increase the gaps at the side portions xe2x80x9cbxe2x80x9d. However, the gap at the central portion is also increased to decrease the output torque.
Thus, in order to increase the output torque by reducing a gap between the central portion xe2x80x9caxe2x80x9d of each pole and the inner periphery of the stator, it has been proposed to set a center of the circular arc of the outer periphery F of each pole to be offset from the center of the rotor, as shown in FIG. 8. In FIG. 8, magnets 1 are embedded in the core 3 of the rotor and the outer periphery F of each pole of the rotor is defined by a circular arc. A center of the circular arc of each pole is offset form the center of the rotor. The circular arcs for respective poles are connected to define the whole periphery of the rotor. According to this arrangement, the gap between the central apex portion xe2x80x9caxe2x80x9d and the inner periphery of the stator is made smaller than the gaps between the outer periphery of the side portions xe2x80x9cbxe2x80x9d and the inner periphery of the rotor. However, with this arrangement, it is not satisfactory to sufficiently increase the output torque and also reduce the inductance of the synchronous motor.
The present invention is to improve a relation between torque and inductance of the synchronous motor, and an object thereof is to provide a rotor capable of increasing the torque and reducing the inductance of the synchronous motor.
A rotor for a synchronous motor of the present invention comprises a plurality of poles and at least part of an outer periphery of one pole of the rotor in a cross section perpendicular to a central axis of the rotor is defined by a curve of a hyperbolic function.
The most part or the whole part of the outer periphery of the one pole of the rotor may be defined by the curve of the hyperbolic function. Specifically, it is preferable to define a central part of the outer periphery of the one pole by the curve of the hyperbolic function.
The hyperbolic function may be expressed as R=Axe2x88x92Bxc2x7(eCxcex8+exe2x88x92Cxcex8), where R represents a distance from a central axis of the rotor or a fixed point, xcex8 represents a rotational angle from a straight line passing through a center of the outer periphery of one pole and perpendicular to the central axis of the rotor, A, B and C are constants and e is a base of natural logarithm or a constant.
Alternatively, the hyperbolic function may be expressed as X=Axe2x88x92B (eCY+exe2x88x92CY) on a X-Y coordinate system with a X axis passing through a center of the outer periphery of one pole of the rotor and perpendicular to a central axis of the rotor, a Y axis perpendicular to the X axis and the central axis of the rotor and an origin as a crossing point of the X axis and the Y axis, where A, B and C are constants and e is a base of natural logarithm or a constant.
In practical machining of the rotor, a region of the outer periphery defined by the curve of the hyperbolic function may be determined based on a train of points on the hyperbolic function curve and a line connecting the train of points by segments of straight lines or curves.